There is nothing in Islam that says that you cannot watch porn In Islam, you cannot have sex with other Muslim married women; for this, you should be stoned to death. If you have sex with unmarried Muslim girl, you will be subjected to 80 lashes. If you do get involved with another man in homosexual acts, you will be forgiven, if you repent.
like I said, just because something is illegal or "not sanctioned" by the religion; it doesn't necessarily mean that its adherants never do it.
I'm pretty sure adultery, sodomy, and fornication are likewise forbidden in most other religions; we still have lots of adulterers, sodomizers, and fornicators, though.